The present invention relates to a method for removing nitrogen oxides from oxygen-containing flue gas steams of industrial combustion processes.
Nitrogen oxides that arise in combustion processes are among the primary causes of acid rain and the related environmental damage. Primary sources of the release of nitrogen oxides into the environment are the exhaust gases of motor vehicles and the flue gases of combustion plants, especially oil-, gas- or coal-fired power plants or stationary combustion engines as well as industrial operations. The nitrogen oxides contained in the exhaust gases consist of up to 60 to 90 vol % nitrogen monoxide, in each case according to the completeness of the combustion process.
One characteristic of the flue gases from these processes is their relatively high oxygen content, which impedes the reduction of the nitrogen oxides contained in them. The excess air coefficient lambda(xcex) is frequently used to characterize the oxygen content. This is the air/fuel ratio of the combustion process normalized to stoichiometric ratios. In the case of stoichiometric combustion the excess air coefficient is equal to one. In the case of superstoichiometric combustion the excess air coefficient is greater than 1xe2x80x94the resulting exhaust gas is lean. In the opposite case one speaks of a rich exhaust gas.
A method that has long been used to remove nitrogen oxides from such flue gases is the so-called selective catalytic reduction (SCR) with ammonia on a specially designed reduction catalyst. Suitable catalysts for this are, for example, described in the patents EP 0367025 B1 and EP 0385164 B1. They consist, for example, of a mixture of titanium oxide with oxides of tungsten, silicon, vanadium, and others. Likewise known are catalysts based on zeolites exchanged with cooper and iron. These catalysts develop their optimum activity at temperatures between 300 and 500xc2x0 C. and a mol ratio between the reducing agent ammonia and the nitrogen oxides from 1.6 to 0.6.
Because of the frequently very high sulfur content of the flue gas that is to be treated there is the danger of poisoning of the catalysts by sulfur. For this reason a flue gas desulfurization plant is frequently connected ahead of the catalytic reduction, although it is operated at temperatures under 300xc2x0 C., so that the flue gas has to be again raised to the operating temperature of the nitrogen removal catalyst before selective catalytic reduction.
An object of this invention is to provide an alternate method for removal of nitrogen oxides from flue gases that additionally enables the production of ammonia as product of value, with simultaneously improved efficiency.
The above and other objects of the invention can be achieved by a method that comprises:
a) treating the flue gas in an electrical gas discharge,
b) passing the thus treated flue gas over a basic storage material for storage of nitrogen oxides in the form of nitrates and discharge of the treated flue gas to the environment, and
c) regeneration of the storage material after its storage capacity has been depleted, by removing the storage material from the flue gas stream and treating it with a reducing regenerative gas stream while forming ammonia.
This invention carries forward from the method disclosed in DE 198 19 372 A1 for reducing the nitrogen oxide content of the exhaust gas of an internal combustion engine. In accordance with this method, the lean exhaust gas of a superstoichimetrically operated combustion engine is passed through an electrical gas discharge plasma. The electrical gas discharge plasma is generated in a so-called plasma reactor. Oxidation of the nitrogen monoxide to nitrogen dioxide takes place in the gas discharge plasma and, employing the water content of the waste gas, nitric acid also forms there. Nitrogen dioxide and nitric acid are chemically bonded in the form of nitrates by a basic storage material and thus removed from the exhaust gas stream.
After depletion of the storage capacity of the storage material, the stored nitrogen oxides, in accordance with DE 198 19 372 A1, are broken down to nitrogen oxides by treating them with a rich exhaust gas and then reduced to nitrogen on a catalyst, where the hydrocarbons contained in the rich exhaust gas are employed as reducing agent for the nitrates. The storage material that is regenerated in this way can be reused for storage of nitrogen oxides in the form of nitrates.
The oxides of alkali and alkaline earth materials, for example, are suitable as a basic storage material, but rare earth oxides can also be used, especially cerium oxide and lanthanum oxide. These materials could be used directly. However, they are preferably deposited in a finely divided form onto high-surface-area carrier materials such as activated aluminum oxide in order to make available an area for interaction with the exhaust gas that is as large as possible.
It was now found that in the regeneration of the storage material with a very rich gas mixture the released nitrogen oxides can be reduced to ammonia. For this the lambda values of the gas stream that is used for regeneration must be under 0.8. In contrast, with the usual regeneration with lambda values between 0.9 and 1.0, only a release of the stored nitrates in the form of nitrogen oxides is achieved.
The storage material for storage of the nitrogen dioxide formed in a plasma reactor, or the nitric acid, can be arranged in the plasma reactor itself or in an absorption vessel situated in the direction of flow of the flue gas beyond the plasma reactor. If the storage material is arranged in the gas discharge, the process steps (a) and (b) run parallel with each other. The gas discharge and storage material can also be arranged in succession in the direction of flow of the flue gas in a single reactor.
In accordance with the invention the storage material is continuously removed from the flue gas stream and sent to a separate station for regeneration of its storage capacity. In this separate station the storage material is regenerated by treatment with a rich gas stream. The regeneration is conducted so that the stored nitrates are released in the form of ammonia. The rich gas stream has to have an excess air coefficient under 0.8 for this purpose. It can be generated in a simple way by a substoichiometrically operated combustion.
The regenerated storage material can either be sent back to the plasma reactor or to the absorption vessel, while the released ammonia is separated from the gas stream used for regeneration by condensation and recovered as a product of value.
In a preferred embodiment of the method, sulfur compounds are separated from the flue gas in a flue gas desulfurization plant before the gas goes to the electrical gas discharge. Since the method separates the nitrogen oxide from the flue gas without using a step for selective catalytic reduction of nitrogen oxides, it is also not necessary to raise the temperature of the flue as back to the operating temperature of the selected catalytic reduction after the gas passes through the flue gas desulfurization unit. The formation of nitrogen dioxide and nitric acid in the gas discharge as well as the storage in the form of nitrates on storage material takes place even at the input temperatures of the flue gas desulfurization plant. Because it is no longer necessary to reheat the flue gas, the efficiency of a power plant, for example, is improved.
In another embodiment of the method, it is combined with a conventional SCR process. The ammonia obtained with the method is then used for selective catalytic reduction of the nitrogen oxides contained in another flue gas stream. In this embodiment of the method, thus the flue gas stream of the power plant is divided into at least two partial streams. One partial stream is purified as given by steps (a) through (c), while the second is passed over a catalyst for selective catalytic reduction. At least in part the ammonia released in step (c) is used as the reducing agent.